@ncoghlan, @ericsnowcurrently, @emilyemorehouse: OK, this PR should be big enhancement for the PEP 432 :-)

Previously, _PyCoreConfig "tried" to be isolated from Py_xxx global configuration variables like Py_IsolatedFlag. In practice, it already contained ignore_environment and utf8_mode which duplicated Py_IgnoreEnvironment and Py_UTF8Mode global configuration which can lead to inconsistency.

With this change, the priority becomes more explicit: _PyCoreConfig has now the priority over Py_xxx. To get a smooth transition, _PyCoreConfig are initialized to -1 which means "unset": in that case, the fields are initialized from Py_xxx.

Core config has the priority:

_PyCoreConfig config = _PyCoreConfig_INIT;
Py_UTF8Mode = 0;   /* just for the example */
config.utf8_mode = 1;  /* override Py_UTF8Mode */
Py_Initialize(&config);
# UTF-8 Mode enabled from config.utf8_mode

Backward compatibility:

_PyCoreConfig config = _PyCoreConfig_INIT;
Py_UTF8Mode = 1;
Py_Initialize(config);
# UTF-8 Mode enabled from Py_UTF8Mode

This change simplify and clarify main.c:

• Py_xxx variables are no longer modified when reading the configuration. Reading the configuration must no longer modify Python global variables: only local variables like pymain, config and cmdline
• Unset fields (set to -1) of config are now always initialized from Py_xxx
• Python initialization now always override Py_xxx from the config <= this part is now better defined

This PR also removes the ugly hack added by the commit f2626ce: two int* pointers added to _PyCoreConfig_Read().

+PyAPI_FUNC(_PyInitError) _PyCoreConfig_Read(
+    _PyCoreConfig *config,
+    int *isolated,
+    int *no_site_import);

It was a temporary hack until I make this more brave change.

Write more tests is a good idea! But this change is already big enough, I will write them in my following change. I plan to move all "cmdline" fields into the core config.